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So, there's this question about how players can learn of a creature's damage vulnerabilities and resistances/immunities; the only answer to date is a Ranger's 3rd level subclass option. I'd like to make a homebrew 'Detect Magic' sort-of spell that grants insight into a creature's immunities, vulnerabilities and resistances. How does this look?

Specifically:

  • Should I make the spell 2nd level? The Ranger's ability opens up at 3rd level, and I don't want to undercut an entire (sub)class by granting one of its core features to spellcasters two levels earlier.
  • I've also made this spell slightly weaker than the Ranger's ability. With the Ranger ability, the DM spills all the beans at once; with this spell it's only 1 res./vul./imm. per round per level. Is that a watering-down significant enough to justify 1st level, or are there not enough creatures with extended lists to have a practical effect?

Detect Resistance / Vulnerability

1st-level divination

  • Casting Time: 1 action
  • Range: 60 feet
  • Components: V, S
  • Duration: Concentration, up to 1 minute

You target any creature within range, and dissect it with your mind's eye. When casting this spell, and for every round thereafter while you hold concentration, you learn of one of this creature's resistances, vulnerabilities, immunities and/or legendary traits. The spell dissipates when there are no more traits left to discover, and the caster is aware of this condition.

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, the spell imparts on you the knowledge of one more trait per round for each slot level above 1st.

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    \$\begingroup\$ I would adjust the duration to a minute, sense you might want to use this spell out of combat. \$\endgroup\$ – NeutralTax Feb 18 at 23:27
  • \$\begingroup\$ Is there a reason you've said 10 rounds instead of the usual 1 minute? Additionally, does learning an aspect of the target after the initial casting require no sort of action? Finally, what do you mean by "legendary traits"? This seems very different from resist/vuln/immun \$\endgroup\$ – Medix2 Feb 18 at 23:33
  • \$\begingroup\$ @NeutralTax changed to 1 minute. \$\endgroup\$ – Willibrord Feb 19 at 6:37
  • \$\begingroup\$ @Medix2 I mean things like the Revenant's 'Turning Immunity', or a dragon's 'Legendary Resistance'. \$\endgroup\$ – Willibrord Feb 19 at 6:54
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Almost...

...would be my answer if the question you're asking is "Is this spell balanced".

I homebrew a LOT of spells, and I have yet to have one of them rejected by my DM. The way I always start is to find an existing spell that functions similarly to what my target spell will do. In your case, that target is a new Detection spell. Fortunately for you, there are a variety of spells we can examine, and use as a template.

Template #1: Detect Magic

There are three 1st level 'Detect' spells. All three have a range of 'self (30 foot sphere)', a duration of '10 minutes (concentration)', a casting time of 1 action, and contain this line:

The spell can penetrate most barriers, but it is blocked by 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt.

Next, we tweak. You want your spell to reveal things about your target. This is a similar function to how Detect Magic works:

you can use your action to see a faint aura around any visible creature or object in the area that bears magic, and you learn its school of magic

Based on that, I think it's entirely reasonable for your spell to allow you to use an action to decipher one resistance, immunity, or vulnerability in a visible creature within range. This should be per round as well, because if you have 4 magic items in range, Detect Magic would take a round a piece to determine their schools of magic.

Template #2: Identify

Your target spell is also similar to Identify, which reveals all aspects of an object at once, and it would also make sense to use this spell as a template. However, Identify has a range of touch, casting time of 1 minute, and has a material component that costs 100gp. To me, this seems less desirable than using the first template.

Pro-level Homebrewing:

You could totally combine the effect of Identify's 'reveal all the things' functionality, and the casting time/range/no expensive material component of the Detection spells....but now you're definitely talking about a 2nd level spell. If you combine the effects of two first level spells, the only way to keep the spell balanced is to make it at least one level higher. Also, I'd suggest giving this spell the ritual tag. This lets you leverage a longer casting time to save spell slots if you need to.

Comparison to the Ranger ability:

This spell is still not nearly as powerful. I've emphasized the key reasons why from the Monster Slayer Ranger's Hunter's Sense ability description:

At 3rd level, you gain the ability to peer at a creature and magically discern how best to hurt it. As an action, choose one creature you can see within 60 feet of you. You immediately learn whether the creature has any damage immunities, resistances, or vulnerabilities and what they are.

The Ranger ability has twice the range, and learns all about the target at once. This is slightly better than what I suggest in the Pro-level Homebrewing section above. That version of the spell would need to be 2nd level, which would require the caster to be the same level as the Ranger in order to access the spell. This is why I think either template version makes sense as a 1st level spell - it's just not nearly as good as the Ranger ability.

The Spell:

I'd say that a balanced homebrew spell that does what you want would look like:


Detect Resistance / Vulnerability

1st-level Divination

  • Casting Time: 1 Action
  • Duration: Concentration, up to 10 minutes
  • Range: 30 feet

You target any creature you can see within range, and dissect it with your mind's eye. When casting this spell, and for every round thereafter while you hold concentration, you can use an action to learn one of this creature's resistances, vulnerabilities, or immunities. The caster is aware when there is nothing left to learn about the creature.

The spell can penetrate most barriers, but it is blocked by 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt.

At higher levels: When you cast this spell using a spell slot of 2nd level or higher, you learn one additional trait per round for each slot level above 1st.

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  • \$\begingroup\$ Don't you mean Range 30ft? The way the header is framed it is an AoE spell but in the description you allude it is single target. If it is AoE a 30ft not sure that a 1st level spell should have that kind of area. \$\endgroup\$ – Slagmoth Feb 19 at 12:47
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    \$\begingroup\$ Maybe a comparison against the ranger's ability to do something like this would help. And consideration of whether or not a 1st level spell doing the same thing is reasonable. \$\endgroup\$ – NautArch Feb 19 at 12:54
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    \$\begingroup\$ @Willibrord: W the Detect spells, like Det Mgc, you can sense magic immediately, but if you want to determine the school of magic, you need to focus (and use an Action) for a round. The other option is Identify as I mentioned, but any time you add things (not having to touch it, no expensive material component, etc), you need to up the spell level to keep it balanced. \$\endgroup\$ – aaron9eee Feb 19 at 13:17
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    \$\begingroup\$ @NautArch added comparison. Thanks for pointing that out!! \$\endgroup\$ – aaron9eee Feb 19 at 13:47
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    \$\begingroup\$ That may be true but I think your wording could use some tweaking as I see this as the trap to which the devs have repeatedly fallen victim. Also, this spell is potentially much more impactful than Detect Magic. \$\endgroup\$ – Slagmoth Feb 19 at 14:06

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